The present invention relates to a method and apparatus for operating an electrochemical fuel cell, such as, for example, to increase the life or durability of the cell.
Electrochemical fuel cells convert fuel and oxidant fluid streams to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. An electrocatalyst is typically disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
In typical fuel cells, the MEA is disposed between two electrically conductive separator plates. A fluid flow field provides a means for directing the fuel and oxidant to the respective electrocatalyst layers, specifically, at the anode on the fuel side and at the cathode on the oxidant side. A simple fluid flow field may consist of a chamber open to an adjacent porous electrode layer with a first port serving as a fluid inlet and a second port serving as a fluid outlet. The fluid flow field may be the porous electrode layer itself. More complicated fluid flow fields incorporate at least one fluid channel between the inlet and the outlet for directing the fluid stream in contact with the electrode layer or a guide barrier for controlling the flow path of the reactant through the flow field. The fluid flow field is commonly integrated with the separator plate by locating a plurality of open-faced channels on the faces of the separator plated facing the electrodes. In a single cell arrangement, separator plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide structural support for the electrodes.
The fuel stream directed to the anode by the fuel flow field migrates through the porous anode and is oxidized at the anode electrocatalyst layer. The oxidant stream directed to the cathode by the oxidant flow field migrates through the porous cathode and is reduced at the cathode electrocatalyst layer.
Solid polymer fuel cells generally use fuels, such as, for example, hydrogen or methanol, which are oxidized at the anode to produce protons. The protons migrate through the ion-conducting electrolyte membrane and react with an oxidant such as oxygen in the air at the cathode to produce water as a reaction product.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate can serve as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell.
Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is typically held together in its assembled state by tie rods and end plates. Apart from being provided with inlets and outlets for the oxidant and fuel streams, the fuel cell stack is typically also provided with a coolant inlet and outlet for the flow of a coolant through the stack.
During operation of a fuel cell various failures or problems can occur which limit the useful life or durability and ultimately the reliability of the cell. For instance, leaks may develop in the ion-exchange membrane (allowing the fuel and oxidant reactants to transfer over to the wrong electrodes) or in the various other fluid seals in the fuel cell. Failures may also occur due to build up of contaminants that collect in the fuel cell.
The types of failure which occur, resulting in declining performance of the cell or failure or breakdown of the cell, and the average time period within which such failure or failures occur can be determined experimentally for a particular type of cell or for a selected number of such cells and then averaged. This time period, whether determined for a single cell or for a selected number of cells, for which the mean value (sometimes referred to as the average value) is then obtained under conventional operating conditions is referred to herein as xe2x80x9cmean life expectancyxe2x80x9d or xe2x80x9cmean time to failurexe2x80x9d (MTTF).
An improved method operates a fuel cell or a fuel cell stack supplied with a fluid stream. The fuel cell has a mean life expectancy that may be determined empirically. The method comprises the step of reversing the direction of flow of the fluid stream after a time period of operation of the fuel cell, the time period being less than the mean life expectancy of the cell. The time period has a value that is a substantial part of the value of the mean life expectancy. A xe2x80x9csubstantial partxe2x80x9d will typically be more than half of the mean life expectancy but may mean 1% or less of the mean life expectancy. In the present methods, the fluid flow stream is not reversed every few minutes but only after operating a substantial portion of the mean life expectancy. The method is useful in increasing fuel cell life or durability, particularly that of a solid polymer electrolyte fuel cell.
The reversed fluid stream may be either one or both of the fuel and oxidant reactant streams, thereby resulting in a flow reversal of one or both the reactants through their respective reactant flow fields in the fuel cell. Where applicable, the reversed fluid stream may be a coolant stream, thereby resulting in a flow reversal of coolant through a coolant flow field.
Reversing the fluid flow direction may change the location at which the greatest amount of degradation occurs for a given degradation mechanism, thereby delaying the onset of a fuel cell failure. The method may desirably be employed once (for instance, after the fuel cell has been operated for about 75% to about 90% of its mean life expectancy) or multiple times during the life of the fuel cell. However, a modest number of fluid flow direction reversals is preferred (for instance, less than about 10 times during the life of the fuel cell).
The fuel cell typically has a port at each end of the flow field for the fluid. The fluid stream is typically supplied to the fuel cell by a supply conduit connected to a first port (inlet) on the fuel cell. The fluid stream is typically exhausted from the fuel cell by an exhaust conduit connected to a second port (outlet). However, certain embodiments may simply vent the fluid exhaust stream (for example, air oxidant) to the surrounding atmosphere. Further, certain embodiments may dead-end the fluid flow field (for example, pure hydrogen fuel).
The flow of the fluid stream may be reversed simply by switching the inlet/outlet functions of the ports, for example, by disconnecting the supply and exhaust conduits from the first port and second ports, respectively, and then connecting the supply and exhaust conduits to the second and first ports, respectively. This may be performed manually or using an appropriate automated subsystem. Where appropriate, fluid flow may be exhausted or dead-ended at or beyond the second port instead. The fuel cell may be designed such that it is symmetric about the first and second ports in which case the fluid flow may be reversed by rotating the fuel cell to align the second and first ports with the supply and exhaust conduits respectively after the disconnecting step.
An improved fuel cell assembly comprises a fuel cell, a fluid supply conduit for supplying a fluid stream to a first fluid port on the fuel cell, a fluid exhaust conduit for exhausting the fluid stream from a second fluid port on the fuel cell, and a fluid stream flow switch for reversing the direction of flow of the fluid stream after a time period of operation of the fuel cell wherein the time period is less than the mean life expectancy of the fuel cell and is equal to a substantial part of the mean life expectancy.
The fluid stream flow switch may reverse the direction of flow of any or all of a fuel stream, an oxidant stream, or a coolant stream. The fluid stream flow switch may be manually activated or the assembly may additionally comprise a controller for activating the fluid stream flow switch. The controller may activate the flow switch automatically.